This invention relates to a planar carbon segment commutator for use with brushes which bear axially against planar contact surfaces of the commutator, instead of bearing radially as in the case of a cylindrical commutator.
It is known, for example from EP 0583892, to provide a planar commutator in which a plurality of commutator terminals are mounted on a commutator base and overmoulded with carbon segments. However, the terminals of these known planar commutators each have tangs to which the armature winding of an electric motor has to be connected.
A number of known methods for effecting such connections are in popular use. Where the winding is formed of low temperature wire, it is usual to employ a soft solder and flux method. Alternatively a cold crimp onto wire that has been stripped of insulation is used in order effect a connection. When dealing with high temperature wires it is necessary to apply heat, and also possibly to apply flux so as to remove the coating of insulation from the ends of the wire.
However, there are a number of inherent problems and undesirable side effects associated with all of the foregoing methods.
Heat causes embrittlement of the copper wire which is used for most armature windings and encourages rapid oxidation. The use of heat also demands a strong structure to support the commutator in order to minimize plastic distortion. This requirement usually demands the use of high temperature compression grade molding material. A further common problem is caused by the accidental stripping of insulation during winding of the armature which is often automated. As the wire passes over the metal of the commutator damage can be caused to the wire insulation and such damage will often be manifest as a short circuited winding. Additionally, there is always a danger of slack in the winding wire causing fretting under the acceleration due to centrifugal and inertial forces.
According to the present invention there is provided a planar carbon segment commutator comprising a commutator base of insulating material, the base having a rotational axis, front and rear surfaces, extending, at least in part, transversely to the rotational axis, and a plurality of first apertures extending through the base, a plurality of commutator terminals each of which comprises a terminal portion and a contact portion, the contact portion of each terminal extending through a respective first aperture in the base and being bent to lie against or in close proximity to the front surface of the base and the terminal portion of each terminal having two cutting edges for cutting insulation on a connector portion of a winding and a slot which in use straddles and grips said connector portion, and a plurality of carbon segments formed on the front surface of the base and over the contact portions, respectively, of the terminals.
Preferably, the commutator includes a housing having a plurality of housing recesses for receiving respective terminal portions.
Preferably, each housing recess has associated therewith means for positioning connector portions of the winding relative to each recess, the base, the terminals and the housing being such that with a single translational movement of the base relative to the housing, the terminal portions enter the housing recesses, the cutting edges strip insulation from connector portions of the winding and the slots establish and maintain electrical contact with connector portions of the winding by insulation displacement.
Preferably, the base has a cylindrical skirt extending rearwardly of its rear surface for receiving the housing.
Preferably, the front surface of the base has a plurality of recesses and each contact portion overlies a respective recess and has at least one aperture through which material forming a respective commutator segment extends into the recess to assist in anchoring the segment to the terminal.
Preferably, the base has a plurality of second apertures communicating with the recesses and through which material forming the commutator segments extends to assist in anchoring the segments to the base.
Preferably, the base has a plurality of third apertures through which material forming the commutator segments extends to assist in anchoring the commutator segments to the base.